Wind tunnel study of the wind turbine interaction with a boundary-layer flow: Upwind region, turbine performance, and wake region

Abstract

Comprehensive wind tunnel experiments were carried out to study the interaction of a turbulent boundary layer with a wind turbine operating under different tip-speed ratios and yaw angles. Force and power measurements were performed to characterize the variation of thrust force (both magnitude and direction) and generated power of the wind turbine under different operating conditions. Moreover, flow measurements, collected using high-resolution particle-image velocimetry as well as hot-wire anemometry, were employed to systematically study the flow in the upwind, near-wake, and far-wake regions. These measurements provide new insights into the effect of turbine operating conditions on flow characteristics in these regions. For the upwind region, the results show a strong lateral asymmetry under yawed conditions. For the near-wake region, the evolution of tip and root vortices was studied with the use of both instantaneous and phase-averaged vorticity fields. The results suggest that the vortex breakdown position cannot be determined based on phase-averaged statistics, particularly for tip vortices under turbulent inflow conditions. Moreover, the measurements in the near-wake region indicate a complex velocity distribution with a speed-up region in the wake center, especially for higher tip-speed ratios. In order to elucidate the meandering tendency of far wakes, particular focus was placed on studying the characteristics of large turbulent structures in the boundary layer and their interaction with wind turbines. Although these structures are elongated in the streamwise direction, their cross sections are found to have a size comparable to the rotor area, so that they can be affected by the presence of the turbine. In addition, the study of spatial coherence in turbine wakes reveals that any statistics based on streamwise velocity fluctuations cannot provide reliable information about the size of large turbulent structures in turbine wakes due to the effect of wake meandering. The results also suggest that the magnitude of wake meandering does not depend on turbine-operating conditions. Finally, the suitability of the proper orthogonal decomposition for studying wake meandering is examined.

A New Miniature Wind Turbine for Wind Tunnel Experiments. Part I: Design and Performance

“…The ratio of the miniature turbine frontal area to the wind tunnel cross-sectional one is less than 0.004, so blockage effects are negligible. More information about the wind tunnel can be found in the previous works of the authors (e.g., [18,19,21]). …”

“…In the light of the above-mentioned benefits of wind tunnel experiments, they have been widely used in the wind-energy community to study wind turbines and their wakes (e.g., [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]). However, …”

A New Miniature Wind Turbine for Wind Tunnel Experiments. Part II: Wake Structure and Flow Dynamics

“…However, the interaction of turbine wakes with boundary-layer flows has gradually received more attention in the wind energy community as wind turbines operate in the atmospheric boundary-layer (ABL) flow. In particular, during the last ten years, wind tunnel experiments (e.g., [8][9][10][11][12][13][14][15][16][17][18][19]) have been performed to study wakes of down-scaled wind turbines in boundary-layer flows. These works sought to provide a better understanding on turbine wake characteristics.…”

Instantaneous Response and Mutual Interaction between Wind Turbine and Flow

“…the engineering model by Jensen [3], experimental study by Bastankhah and Porté-Agel [4], or detailed numerical investigation on the wake structure and breakdown by Sørensen et al [5].…”